1. Field of the Invention
The invention relates to a method for reducing unwanted substances by injecting a reactant into a flue gas of a steam generator, in which the reactant is injected into the combustion chamber of the steam generator via a reactant opening of a multi-component nuzzle. The invention further relates to a multi-component nozzle for injecting reactant into a combustion chamber of a steam generator for reducing unwanted substances in the flue gas, having a reactant feed for feeding a reactant, wherein on the combustion chamber side the reactant feed opens into a reactant opening. Finally, the invention also relates to a combustion chamber of a steam generator having such a multi-component nozzle.
2. Description of Related Art
Methods and devices of the kind previously mentioned are already known. The reactants are, for example, ammonia and/or urea, which can reduce the proportion of nitrogen oxides in the flue gas. Corresponding methods are labelled selective non-catalytic reduction (SNCR). Typically, the ammonia and/or urea is/are injected in an aqueous solution at one place into the combustion chamber, where a temperature between 850° and 1000° prevails and one of the following reactions takes place:4 NH3+4 NO+O2→4N2+6 H2ONH2CONH2+2 NO+½ O2→2 N2+CO2+2 H2O
Different nozzle types are used for this purpose, which are either integrated into the wall of the combustion chamber or project into the combustion chamber in the form of lances. The lances can carry a nozzle fitting, so that the reactant can not only be added at one point but also distributed over the cross section of the combustion chamber by means of a plurality of openings. The use of corresponding lances has the drawback of ash deposition and corrosion, since the lances are exposed to the flue gas and the high flue gas temperatures. In addition, with larger combustion chambers very long lances and elaborate nozzle fittings are required in order to mix the flue gas with the reactant satisfactorily, without, for instance, accepting an increased ammonia slip. Long lances can also be induced into unwanted vibrations. In addition, the lances have to be reliably cooled. Furthermore, when aligned vertically they must be guided through heat exchangers provided above the combustion chamber, so that retrofitting existing equipment due to decreasing limit values is barely possible. Although horizontal and vertical lances can be removed from the combustion chamber for maintenance purposes, provided that they are tubular, they then cover a small cross section of the combustion chamber.
Nozzles embedded in the wall of the combustion chamber have the drawback of a low penetration depth. This can be increased by using so-called two-component nozzles. Typically, ammonia and/or urea in aqueous solution is/are fed to the two-component nozzles. The solution is then delivered into the combustion chamber via a central opening.
An annular opening is provided concentrically around this opening, through which a propellant is additionally injected. Gases, in particular air, are mostly used as the propellant. The air discharges so fast from the nozzle that it carries the reactant into the combustion chamber along with it and nebulises the reactant. Hence, fine droplets of the reactant are formed.
Although the penetration depth in the case of two-component nozzles is greater with liquid reactants than if gaseous ammonia were injected, the penetration depth for large steam generators or large combustion chambers is not sufficient, since the fine droplets of the reactant are carried along by the flue gas and are quickly vaporised due to the high temperature. If the discharge speed of the propellant is increased, the speed of the droplets can also be increased, which should theoretically increase the penetration depth. However, the droplet size then also decreases and more flue gas is sucked in by the propellant jet, so that the droplets vaporise more quickly and thus the penetration depth cannot, in fact, be appreciably increased. Therefore, the reactant only comes close to the middle of the combustion chamber in the case of smaller combustion chambers, so that the flue gas can come into contact with the reactant over the whole cross section of the combustion chamber. Two-component nozzles can also be attached to lances in the combustion chamber but are then also exposed to depositions and corrosion.